1. Technical Field
The invention relates to a spectrometer. Furthermore, the invention relates to a method for the wavelength calibration of echelle spectrometers.
Echelle spectrometers are spectrometers operating wi have a high angular dispersion, i.e. the ability to angularly separate closly proximate wavelengths. This has the advantage of a high resolution and yet small dimensions of the assembly. Therefore, echelle gratings are particularity suitable for high resolution spectroscopy, such as atomic absorption spectroscopy with continuous light sources. A spectrometer with an echelle grating normally operates in very high diffraction orders. Typical values are 20th to 150th order. The free spectral range in each order is comparatively small.
In order to avoid spectral overlapping of different diffraction orders in the exit plane of the spectrometer, echelle spectrometers are used in combination with an internal separation of the orders which is perpendicular to the direction of the echelle dispersion and leading to two-dimensional spectra. The use of echelle gratings in combination with a pre-monochramator for the separation of the orders is also known as a so called double echelle spectrometer assembly. The radiation is specrally pre-selected for example by means of a prism. Only the radiation from a limited spectral range essentially corresponding to one order enteres the echelle spectrometer. The generated echelle spectrum has a linear spectral form. Diffraction gratings or prisms are used as dispersing optical elements for the selection of the order with a pre-monochromator. The directions of the dispersion of the pre-monochromator and the echelle grating are parallel to each other.
For most applications it is necessary to calibrate the spectrometer. A wavelength is allocated to each geometric position in the exit plane of the spectrometer. The calibration can vary due to temperature changes, vibrations or other mechanical changes. In this case it may be necessary to re-calibrate the device.
2. Prior Art
From the DE 41 18 760 A1 a double spectrometer assembly is known having a fluid prism with variable prism angle which generates a spectrum with a low, adjustable dispersion. The spectrum is imaged on an intermediate slit simultaneously forming the entrance slit, i.e. the field stop for the following echelle spectrometer. The intermediate slit cuts out a partial spectrum from the entire spectrum of the light source to be measured, the spectral band width of such partial spectrum being at least smaller than the band width of the corresponding diffraction order of the echelle grating. Such an assembly operates with a small intermediate slit with constant width. The width of the intermediate slit is selected similar to the width of a picture element of the detector (pixel). The width of the entrance slit is comparatively large. The selection of the spectral band width of the light entering through the intermediate slit is effected by varying the linear dispersion in such a way that the prism angle is adjusted accordingly. The position of the wavelength of the portion of the spectrum is adjusted by rotating the prism. The position of the portion of the spectrum on the detector of the echelle spectrometer is adjusted by rotating the echelle grating. The precision of the adjustment of the wavelength position is determined by the precision of the mechanical adjustment of the angle of the echelle grating and the prism of the pre-monochromator, respectively.
From DE 195 45 178 A1, a spectrometer assembly is known consisting of an echelle spectrometer and a preceding prism spectrometer for the separation of the orders, the assembly using the Neon spectrum of a low pressure discharge lamp as a line source for the wavelength calibration of the echelle spectrometer. The light from the line source enters the echelle spectrometer bypassing the prism spectrometer through an auxiliary slit in the plane of the intermediate slit and is detected with additional detector elements at the light detector. In such an assembly the widths of the auxiliary slit and the intermediate slit are constant, have the same width and they are smaller than the entrance slit of the pre-monochromator. The intermediate slit forms the field stop for the double spectrometer assembly in a known way. The width of the entrance slit of the described assembly can be changed in steps. Using a fixed prism angle the entrance slit serves as the spectral limitation of the light beam entering the echelle spectrometer. The light entering through the auxiliary slit without pre-dispersion for the wavelength calibration generates a characteristic pattern of spectral lines on the detector. Not all the lines belong to one diffraction order of the Echelle grating, but represent the superposition of the different diffraction orders of the grating. Each line exactly represents one pair of values for the incident and diffraction angle at the grating. With a sufficient line density at least on line is imaged on the reference detector for each position of the grating. By mechanically coupling of the detectors on a common silicon chip a wavelength calibration of the measuring detector can be performed for each measuring wavelength using the position of the reference line with the second, parallelly arranged reference detector. The accuracy of the adjustment of the wavelength position is only determined by the measuring accuracy of the measurement of the reference spectrum, apart from the various imaging errors of the measurement and reference spectra. Thereby, it is now independent from the accuracy of the mechanical adjustment of the Echelle grating.
It is a disadvantage of the known assemblies, that each detector element of the light receiving detector is illuminated by the light from a different position of the entrance slit which is significantly wider than the intermediate slit. Thereby a measuring error can be generated, when using the double spectrometer assembly especially for the investigation of light sources with an inhomogeneous light density distribution.
Furthermore, the accuracy of the adjustment of the wavelength position of the wavelength range selected by means of the pre-monochromator is completely dominated by the accuracy of the mechanical adjustment of the dispersing element used for the pre-monochromator. Furthermore, the line density of the calibrating light source often is not sufficient with very high linear dispersion of the echelle spectrometer to image at least one calibration line on the detector for each grating position.